Keywords

1 Introduction

Textile industries have been present since civilization and subsequently have been growing and proportionally blooming, along with the increasing fashion trend through time [1]. The waste components from the textile industry starting with the natural compounds as the chief raw material, which include both toxic and safe natural colorants derived from plants, insects, mollusc shells, and natural-colored minerals [2]. The waste thus produced from the textile productions can be considered negligible and are easily removed or treated with dilution and natural neutralization. But, since the introduction of the chemically engineered colorants and dyes, subsequently increasing the utilization of such cheaper and stronger dyes which replaced the natural dyes [3]. Synthetic dyes are classified into different groups based on their chemical makeup. Extensive studies on structure and toxicity of most of the dyes are well understood for various applications [4]. The contamination of synthetic dyes and colorants causes negative effects on environment and health [5]. For instance, presence of any colored inclusion in the aquatic system blocks sunlight penetration that further damages the aquatic ecosystems [6]. Chemically synthesized dyes are very complex in nature, stable, and last for an extended duration influencing the natural water system negatively [7]. The presence of industrial dyes and organic synthetic dyes in the environment leads to various health issues and it linked to many illnesses ranging from organ damage to cancer [8].

Conventional methods including chemical and biological wastewater treatment methods are used for the remediation of such textile wastes, unfortunately the synthetic organic dyes are unable to be removed by conventional treatment techniques. Even some of the synthetic dyes and industrial colorants are very lethal to the microorganisms which are involved in biological treatment methods due to their complex and toxic structure [9]. Under the chemical treatment process, more chemical additives are introduced to remove the waste in solid form, which does not work for many water-soluble dye molecules [10]. Membrane filtration is the final step in removal of the leftover dye molecules. Reverse osmosis is usually preferred for its ability to remove any material from the effluent water, but the process usually is energy-consuming and more than 75% of the water is rejected back as a concentrated solution [11]. The conventional water treatment process is energy and resource consuming and is not sustainable in the long run [12].

Sunlight-driven treatment processes such as catalyst-based photo-degradation is an alternative and potential method to treat such toxic dyes and industrial colorants. Among all renewable energy Sun is the basic source of energy for life as we know to exist and is renewable. Catalysts are a group of emerging materials, which are capable of initiating the degradation reaction under light. Photocatalysis can be a better solution for the demineralization of various textile dyes and offers a better alternative to conventional methods [13]. There are different kinds of nanocatalysts with desired properties which can be used for treatment of dyes at different levels [14]. Metal–organic framework materials are found to have a greater surface area with the presence of pockets that enhances the absorption and adsorption process in waste removal, having heavy metals and other toxic ions usually adsorbed. The presence of fibers with defined shape and structure is very good at selective filtration under modification. This leads to the introduction of nanofilters, a selective membrane that mimics the biological membrane in functionality [15]. Then comes the more sustainable and efficient nanomaterial for waste treatment nanophotocatalyst, which is from carbonaceous nanomaterials to inorganic metal oxide nanomaterials. A nanophotocatalyst is capable of demineralization of various textile industrial dyes in presence of a light source [16].

2 Classification and Toxicity of the Textile Dyes

Dyes are vaguely classified into natural and synthetic, the natural dyes are derived from the natural world such as plant, animal, insects, and even minerals [17]. A natural dye is usually very laborious with a high price tag and often the dye molecules are not stable and are easily removed or degraded under the washing and drying process [18]. Now natural colors are limited in food industries mainly and but not in textile industries. On the other hand, synthetic organic dyes are brighter and resistant to degradation and are available at a cheaper rate, making them a perfect fit for the textile industry [19]. The synthetic dyes are further subdivided into different categories based on the presence of chromophore groups in their structural framework, such as azo dyes, anthraquinone, etc. [20] or as acidic and basic as per their dissociation property in water [21].

Among all the groups mentioned, the most prominent and evasive dyes used across the globe are the aromatic azo dyes [22]. The toxicity of these dyes is more or less well known to science. Natural dyes can be considered safer compared to synthetic ones [23] with several exceptions. Certain natural dyes are retrieved from very toxic plants and ingestion of such dye is lethal [24]. The presence of such dyes bears both ecological and health deterioration effects. Many of these dyes are persistent and remain for a long duration with increased negative impacts on environment and human health [25]. The textile dyes in an aqueous medium tend to present themselves in either acidic or basic contributing to pH fluctuation in the water body and lethal to the aquatic animals [26]. Many of the dyes from the azo group are considered to be highly toxic for both environment and human, and exposure to such chemicals is found to cause skin disease to organ failure and cancer [27]. A small quantity of organic dye is enough to color the water bodies and presence of a colored compound in water can limit the penetration of sunlight for aquatic fauna. In short, untreated dumping of dyes and colorants from the textile industry certainly causes an impact on the environment and the organisms. The schematic in Fig. 1 illustrates the influence of textile dyes in the environment [28].

Fig. 1
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Schematic representing influence of textile dye on environment

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3 Phytoremediation

Plants and other photosynthetic organisms are the pillar of any ecosystem. They are naturally existing components capable of utilizing solar energy and converting it into chemical energy [29]. Phytoremediation is vastly studied and used in many places for remediation, such as heavy metal removal from the soil by growing plants such as cabbage. Phytoremediation can be performed with plants and also with phytoplankton [30]. The process of remediation of textile dyes using photosynthetic organisms is through different modes starting with adsorption, absorption, and enzymatic digestion, starting from the root hairs to other tissue parts in plants and through the cell itself in phytoplankton [31].

3.1 Phytoremediation with Plants

Plants are the multicellular organism with defined tissue structures. This photosynthetic mass is capable of absorption, digestion, and another process with the sun as the source of energy [32]. The plants produce different proteins and enzymes which are considered as naturally existing nanomaterials capable of catalysis [33]. Many studies have been carried throughout the decade to provide plants, which are easily available, cheaper, safer, and sustainable method for remediation of pollutants like synthetic dyes, heavy metals, etc. Usually, the setup for phytoremediation process has a different process, such as phytoextraction, phytofiltration, and phytostabilization, which is carried out for removal of inorganic metals from the waste solution in the soil [34]. Figure 2 gives a summarized view of processes involved in phytoremediation.

Fig. 2
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Process involved in phytoremediation of dye waste effluent

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3.1.1 Phytotransformation

A process where the plant absorbs dye molecules from the soil with water molecules and then converts them into less toxic substances through enzymatic action [35]. Eichhornia crassipes, a wetland plant is shown to remove 95% of colorant Black B and Red RB through adsorption and it reduces the toxicity level of the dye molecules. Some plants used for this phytotransformation are Lemna minor, Pistia stratiotes, Typha angustifolia, Typhonium flagelliforme, Nasturtium officinale, and Azolla filiculoides, etc. are found to be efficient in adsorbing different dye molecules from the aqueous solution and also capable of degrading the dyes with enzymes such as laccases, tyrosinases, azo-reductases, 2,6-dichlorophenol indophenol reductases, etc. which will reduce the BOD and COD providing stability to the water body [36].

3.1.2 Rhizo-degradation

A process where a plant sends a chemical signal through the root to microbes, which breaks down the organic molecules. This is a synergistic method of degrading complex organic molecules [37]. The humus layer of soil is filled with microorganisms and many form synergistic relationships with the plant root. The in vitro synergistic effect of such relation was seen in Gaillardia pulchella (plant) and Pierre monteilii (bacteria), where 100% degradation of the textile effluent was reported [38].

3.1.3 Phytovolatilization

It transforms the organic molecules into gaseous molecules and ejects out through respiration [39]. Many plant species are capable of phytoremediation without having any negative influence on the plants. Typhonium flagelliforme is one such plant that is capable of degradation of dye molecules from the water system even when the water has no nutritional content [40]. A study conducted for remediation of waste effluent with mono- and di-sulphonated anthraquinone was carried using Rheum rhabarbarum was found to be efficient under hydroponic setup. Certain species of Aloe vera was found to degrade toxic colors like malachite green and Congo red [41].

4 Phytoplankton for Phytoremediation

Phytoplanktons are a group of photosynthetic organisms made up of single cells or few groups of cells [42]. These are mainly composed of different algal groups. The potential benefits of using algae over larger plants are less requirement of land, ease of growing, more efficient at degradation, and rapid growth under nutrient-rich soil or solution with enough sunlight [43].

Algae are divided into macroalgae and microalgae, based on their size. Both algae are efficient in phytoremediation [42]. Lab-scale studies have proven macroalgae to be efficient in the demineralization of dyes. Macroalgae like Streptomyces glaucescens and Solanum marginatum where adsorption is studied for degradation of dye effluent and is found to remove diazo dye effectively. Many other species of macroalgae are also efficient in the removal of dye from waste effluent. Biosorption using microalgae is studied for the same purpose of removal of the dye from waste effluent. Acutodesmus obliquus and Chlorella vulgaris are species of microalgae that have shown removal efficiency of 44 mg/g and 53 mg/g of acid red dye and remazol golden yellow dyes, respectively [44]. The process of phytoremediation with algae is dependent on the solution pH. In lower pH, the organic materials get protonated and attain a positive charge, which enhances the adsorption process as the negative cell wall will attract the positively charged organic particles. The reverse happens when the pH is increased and the adsorption decreases as well [45].

5 Nanocatalyst-Based Photocatalytic Degradation of Textile Dyes

One of the most sustainable treatment methods of industrial dyes and colorants is nanoparticle-based photocatalysis. Nanoparticles are any material whose size is less than 100 nm in any one direction [46]. The material, when in nanometre, functions and presents characteristic capacity which is usually not associated with bulk or atomic element or compound [47], such nanomaterials possess enhanced optical and electrical properties [48]. Nanoparticles are comparatively stable compared to organic and inorganic catalysts. They have high surface area that contributes to increased adsorption and reaction area. Most of these materials are capable of catalytic reaction when exposed to light [49, 50]. This property can be enhanced by doping the nanomaterial with ionic metal or other nanomaterials which can be used in the various fields from sustainable energy to sustainable waste management [51]. This chapter will discuss materials for textile dye degradation with solar energy as the driving force rendering the process sustainable and effective as well. The nanomaterial is categorized into organic and inorganic at broad and further subdivided into a different group based on their chemical and structural makeup. Such different compositions and morphologies play an important role in their function as photocatalysts. The most well recognized and studied nanomaterial for photocatalysis are metal oxides due to their efficiency with easy synthesis and safety [52].

The principle behind photocatalysis of textile dye using nanophotocatalyst is based on the excitation of the particle under exposure to the sun. The material absorbs an amount of photon, which on excitation allows for an electron to jump from the valence band to conduction band creating an electron–hole pair, which in presence of the aqueous solution produces intrinsic hydroxide and oxygen radicals that aids in the degradation of the organic dye molecule [14]. Figure 3 summarizes the basic mechanism of photocatalysis.

Fig. 3
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Schematic representing mechanism involved in photocatalysis

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5.1 Metal Oxide Nanoparticles

The metal oxide nanoparticles are one of the earliest discovered and studied materials for their catalytic property in the field of electronics and sustainable material. Metal oxide nanoparticles are easier to synthesize compared with other complex materials [53]. Such materials are synthesized using the salt of the respective metal oxide. Titanium dioxide was the first nanomaterial discovered as a photocatalyst. Titanium dioxide is widely studied and used for remediation process, but due to their high bandgap of 3.2 eV they can effectively use only 4% of the solar spectrum, hence doping is carried out which in turn reduces the bandgap and allows photocatalysis under visible light as well [14]. Titanium dioxide is doped with organic materials like polyacrylamide, chitosan, and other carbonaceous materials for improvement [54]. Much research follows the variation in synthesis pattern to increase the visible light absorption. Tin oxide, zinc oxide, copper oxide, and iron oxide are a few metal oxides that are used for solar-driven photocatalysis for remediation of textile dye effluents [55]. Other metal oxides like zinc, copper, and manganese can also be tuned as functional catalyst under both UV and visible light with different methods of synthesis and doping [56].

5.2 Chalcogenide

Chalcogenides are complex cations along with elements from groups of 16 in the periodic table. Elements like oxygen, sulfur, selenium, tellurium, and polonium are branded as Chalcogen [17]. These materials are well studied and used in the field of optoelectronics. They are also excellent photocatalyst with most of them having a small bandgap within 1–2, making them a perfect photocatalyst for degradation of a pollutant under sunlight [57]. Layered metal chalcogenide is a very efficient heterogeneous catalysis and is found to degrade textile dyes efficiently. TiSx, AgSX are some metal chalcogenides which are used in the degradation of textile dye effluents [58].

5.3 Metal–Organic Framework (MOF)

A metal–organic framework is a group of nanoparticles that are also known as porous coordination polymers and are gaining interest over its highly porous and diverse structural property. Due to the presence of pores the surface area increases, which can increase the adsorption site of the material. This material is of great interest for storage of gases like hydrogen, or membrane filtration, sensors, or in drug delivery. MOF material is a photocatalyst that is mainly functional under visible range and UV light [59]. A metal–organic framework is one of the most efficient materials for photocatalytic reduction of gases and water into organic fuels and hydrogen fuel, which are the future of the energy industry. Due to such capability, it is also widely studied in degradation of dynamic dyes released from the textile industry [60]. Zr Porphyrin metal–organic framework has been used in the removal of the dyes and used as a self-cleaning material under solar irradiation. Cu- and Zn-based metal–organic frameworks are some of the most efficient and studied materials capable of improved dye degradation. Certain studies show a degradation percentage of above 90% within a very short duration [61]. The benefit of this material over metal oxide is that it is composed of both organic and inorganic materials with increased surface area and characteristic function with reduced bandgap with functionality spreading into visible to far-infrared spectrum increasing catalytic efficiency [62]. Further doping with nanoparticles can enhance its catalytic efficiency in the degradation of organic pollutants.

5.4 Carbonaceous Nanophotocatalyst

Carbonaceous nanomaterial is an organic structure derived from different organic precursor chemicals like urea, graphite, melamine, proteins, etc. and also from natural plant parts like leaf stems and barks [63]. Most of such material is either polymeric sheet or hollow tube in structure with high surface area for adsorption [64]. These materials contain great conductors of electricity unlike their bulk counterparts, e.g., graphene [65]. Usually, such materials are used along with certain metallic nanoparticles to function as an efficient catalyst but through much research. Different carbonaceous nanomaterials which are capable of photocatalysis without any dopant have also been developed [66]. These advanced materials are of great use in the sensor applications for environmental monitoring to optoelectronics and photocatalysis. Carbon nitride, graphene oxide, etc. are several engineered carbonaceous materials that are capable of demineralization of textile dyes under sunlight. Band structure engineering of graphitic carbonitride deems the material as a great material in the degradation of dye under solar irradiation [67]. Carbonaceous materials are safer for the environment and human health comparatively as it is organic based [68].

5.5 Perovskite

A perovskite is a group of material having formulae of ABO3, where the A and B are metals [69]. The perovskite material is a very well-established material in optoelectronics application for its exceptional optical and electrical property [70]. Recent studies have revealed that perovskites are great photocatalysts and have proven very efficient in the degradation of textile dyes. Some examples are degradation of dyes with ZnTiO3, SrTiO3 that is doped to reduce the bandgap, which have been proven as efficient catalysts for degradation of dye under constant solar irradiation [71]. Sometimes the precursor molecule and synthesis procedure can also influence the absorption efficiency such as black SrTiO3 which even is capable of photocatalysis under sunlight with high efficiency [72].

5.6 Quantum Dots

The discovery of quantum dots has been a milestone in the field of optoelectronics due to its small size that provided enhanced optical and electrical property [73]. Besides being used in optoelectronics, they are also a great catalyst and dopant. They are used along with effective nanoparticles to enhance absorption process to enhance degradation of textile dye waste and other organic pollutants [74]. They are used in combination with other nanoparticles like titanium dioxide to reduce the bandgap and increase their activity in the visible light allowing the material to demineralize textile dye waste using the sun as the driving energy [74].

6 Advantages of Using Material Which Uses Photon as the Driving Energy

Depending only on the sun as the major driving energy in degradation of textile dye has both negative and positive influence. The utilization of materials capable of photocatalysis using solar energy allows for a cleaner and greener alternative to the traditional remediation of textile dye. The exhaustion of solar energy won’t be a concern and can provide energy for a prolonged duration. The cost and energy consumption for such a setup will be lower than the conventional method of treatment of textile dye. Conventional treatment plant used both high energy-consuming and polluting chemicals while photocatalysis requires only the catalyst and the sun to complete the remediation process. Phytoremediation is also the most efficient and cheap natural photocatalytic complex system. Throughout its lifecycle, it will keep on absorbing and degrading textile dye from wastewater. Fast reproduction and high metabolism make the phytoplankton an ideal organism for phytoremediation of textile dye-polluted water.

Nanoparticles of all kinds mentioned above are an abiotic catalyst that has been studied and developed over the years of research. Such particles are very efficient photocatalysts due to change in their physical and electrical properties as a result of small size. These materials possess a high surface area, which helps in the adsorption of the dye molecule, which is then degraded under the photocatalytic process. The utilization of nanoparticles for remediation of textile dye is efficient in both degradation and time conservation. Duration required for maximum degradation of textile dye usually takes 2–4 h allowing for efficient degradation of textile dye within half a day. In brief, the utilization of materials that can harness solar energy for remediation of the textile dye pollutant is a very sustainable technique that can provide enhanced remediation of any pollutant with no energy wastage and low capital cost.

7 Disadvantage of Using Sun as the Driving Energy

Even though the utilization of solar energy as driving energy is very sustainable, it is not always efficient. The intensity of solar energy is never constant and is dependent on environmental conditions like weather, duration of day and night, air quality, etc. The sun will be available all day on a summer day but that is not the case in the rainy and winter season. The solar energy will be abundant near to the equatorial region while it reduces as it drifts toward poles making this technology obsolete. The plants take longer duration to completely remove dye from the textile wastewater while the nanoparticles are not well studied at full scale in remediation of textile waste treatment. The nanoparticles performance usually reduces over time due to loss of the material while filtering and washing.

8 Conclusions

Nanocatalyst-based photocatalytic treatment of industrial wastewater has great advantages especially to remove organic dyes and colorants which are not able to degrade by conventional methods. Photocatalytic treatment of textile industrial dyes and colorants is environmental friendly and economic where natural sunlight can be used as an alternative driving energy without production of any secondary pollutants. Recent studies clearly revealed potential removal of various dye molecules using improved nanocatalysts. However, some of the associated problems like designing of affordable nanocatalyst and their availability for real-time applications at commercial level yet to be studied and optimization of photocatalytic process for target dyes need to be done urgently.